Seal by-products (grey seal with the bone in, grey seal with the bone removed, and harp seal) were subjected to five different processing methods: high (100 °C) and low (45 °C) temperature oven-drying, freeze-drying (FD), silage by acid or natural fermentation. Growth performance of diets containing these seal by-products was evaluated in rats as a monogastric model species. With the exception of naturally fermented grey seal without bone, weight gains for rats fed the boneless grey seal products were highest of the seal products (24.65–30.04 g rat^-1) and statistically similar to those of rats fed casein (32.15 g rat^-1). An in vivo crude protein (CP) digestibility study was conducted using 12 adult white rats in metabolic cages that allowed separate collection of urine and feces. The 16% CP diets contained chromic oxide as an inert fecal marker at 0.5%. Total fecal and urine production, as well as feed and water intake, were recorded daily within the three experimental periods. Digestibility of CP was significantly higher for the naturally fermented grey seal without bone silage (94.0%) than casein (89.0%), whereas the other seal products were statistically similar (91.7%–92.7%). The CP content of FD grey seal was as high as 91.7% (FD). Seal by-products have the potential to be used as alternative high-protein feedstuffs in monogastric diets.